1. Field of the Invention
The present invention relates to an image display device and driving method thereof. More specifically, the present invention relates to an organic EL (electroluminescent) display device.
2. Discussion of the Related Art
Generally, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-programs N×M emitting cells to display images. As shown in FIG. 1, a typical organic emitting cell comprises an anode (made of indium tin oxide (ITO)), an organic thin film, and a cathode layer (metal). The organic thin film may have a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL), an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving organic emitting cells include the passive matrix method, and the active matrix method, which uses thin film transistors (TFTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs). The passive matrix method forms crossing cathodes and anodes and selectively drives data and scan lines. The active matrix method couples a TFT and a capacitor to each ITO pixel electrode to maintain the voltage by utilizing the capacitor. The active matrix method includes a voltage programming method or a current programming method, depending upon signal forms supplied for programming a voltage at a capacitor.
FIG. 2 shows a conventional voltage programming pixel circuit for driving an organic EL element.
As shown, the conventional voltage programming pixel circuit comprises transistors M1, M2, M3, and M4, capacitors C1 and C2, and an organic EL element OLED. The data line Dm transmits data voltages for displaying image signals to the pixel circuit, the capacitor C2 is coupled to the power VDD, and a cathode of the organic EL element OLED is coupled to a power VSS. A threshold voltage of VTH at the driving transistor M1 is compensated by select signals provided from three scan lines Sn, AZ, and AZB, and a current corresponding to a data voltage VDATA is controlled to flow to the organic EL element OLED.
The conventional pixel circuit compensates for deviation of the threshold voltage VTH of the driving transistor M1, but requires three additional scan lines for such compensation. This many scan lines may degrade the display device's aperture ratio and provide a complicated driving circuit.